Integrating viral vectors are a good choice for gene therapy because they offer fairly efficient transduction and consistent long-term gene expression. Much research has been directed towards improving vector design to increase safety and reliability. A promising approach is to establish control over the fate of transduced cells in vivo. Incorporating an effective suicide gene into a therapeutic vector can ensure that any malignant clones arising from deleterious insertion of the vector can be specifically killed. Likewise, such a control schema could be used as an inserted safety component for a variety of stem cell transplantations, reducing teratomas, for example, should these outgrowth events develop as occurred in one very recent study2. A suicide gene schema can also be used to control post-transplant complications.
The infusion of donor lymphocytes in allogenic bone marrow transplant (BMT) recipients provides potent antitumor activity to treat recurrent malignancies. One complication, however, is severe GVHD (Graft Versus Host Disease), which is mediated by T cells in the graft. One approach to control GVHD is to employ suicide gene therapy.
Donor T cells mediate both GVHD and a GVL (Graft Versus Leukemia)-effect after allogenic haematopoietic stem cell transplantation (HCT), and the separation of GVL from GVHD has proven to be a formidable problem. The expression of an inducible suicide gene in donor T cells was conceived as a potential way to provide for the abrogation of GVHD after leukemic cells were eradicated. The most extensively studied suicide genes are derived from pathogens and include the HSV-tk and bacterial cytosine-deaminase genes, which encode enzymes that metabolize ganciclovir and 5-FU, respectively, and generate toxic active compounds (Carolina Berger, Mary E. Flowers, Edus H. Warren, Stanley R. Riddel. Analysis of transgene-specific immune responses that limit the in vivo persistence of adoptively transferred HSV-TK-modified donor T cells after allogenic hematopoietic cell transplantation. Blood 2006, 107:2294-302.)
In the customary adaptation of this approach, the herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene, combined with the antiviral prodrug ganciclovir (GCV), is used to control GvHD after introduction of this suicide gene into donor T lymphocytes. However, the efficiency of HSV1-tk is suboptimal and the issue of host immunogenicity against this heterologous effector gene product can hamper outcomes. In addition, prophylactic GCV is often used to control cytomegalovirus infection after BMT. This confounds the broad clinical implementation of this approach.
HSV1-tk mediated cell killing requires cellular proliferation for its cytotoxic effect. This limits the effectiveness of gene therapies employing tk to only dividing cells. Quiescent disease cells will escape destruction and may persist. Tumor cells have been shown to remain quiescent for long periods of time (Trends Cell Biol. 15(9):494-501, 2005).